1. Field of the Invention
The invention relates to an electrical power transfer device like a slipring or rotary joint and brushes thereof. Sliprings are used to transfer electrical power between rotating parts of machines like wind power plants, CT scanners or electrical generators. There a brush, mainly including electrically conductive material like a metal is sliding on a rotating cylindrical track of conductive material like brass, which may have gold plating.
2. Description of Related Art
The European patent application EP 0054380 A1 discloses a slip ring and brush assembly. The brush includes a bundle of thin electrically conducting fibers, which project from a brush holder to contact the slipring. The annular contact surface of the slipring is provided by a gold layer thereon. By making the fibers of a material harder than the gold layer, transfer of gold from that layer to the contacting regions of the fibers during an initial period of use can be encouraged, thereby to improve the subsequent operating characteristics. The disadvantage is comparatively poor electrical characteristics when the brushes are new. When a brush slightly varies its position or orientation, the electrical characteristics are again poor until a gold layer has been established at the new contact point on the surface of the brush.
In the European patent application EP 317 030 A2 a further slipring assembly is disclosed. Here the brushes are wire springs including gold. In an alternative embodiment, the wire springs are of a metal baser than gold and the free ends have a sleeve of gold. The solutions provided herein are comparatively expensive. The wire spring of solid gold requires a large amount of gold, while a sleeve of gold is difficult to manufacture and difficult to apply to the end of the wire spring. Furthermore, the sleeve may be pulled of the end of the wire spring, which results in a complete loss of transmission characteristics.